Two armatures. Two animations. Combine into one armature with both animations intact?
Whoah I had to paste together a bunch of answers online but heres a start to doing just this: 

create a new bone in the armature and use bone constraint to copy whatever attribute you need from the other armature bone. Then in pose mode, do pose -> animation -> bake action and THEN select 

only selected
visual keying
clear constraints
overwrite current action - thats the one that missing from most answers online